Fluorinated sulphonic acids are generally employed as acid catalyst for various reactions including alkylation of hydrocarbons such as isobutane with lower olefins such as propene and butenes as described in U.S. Pat. Nos. 5,220,095, 5,245,100 and 5,498,820. These processes produce typically various amounts of spent acid, in which the fluorinated sulphonic acid is passivated with respect to the desired reactions by by-products formed during the reaction. In order to ensure feasible economics the acid catalyst has to be recovered from the spent acid and recycled to the process.
The acid may be recovered from spent acid by treatment of the spent acid with sulphuric acid (U.S. Pat. No. 5,618,769) or esters of sulphuric acid. The acid may also be recovered by extraction from the spent acid with water, and, subsequently, isolation from the aqueous solution. Recovery of the acid from aqueous solution can either be achieved through distillation of a salt of the acid with sulphuric acid (DK 170,396) or by distillation after addition of a weak base in form of a salt of the acid to the aqueous solution of the acid in order to facilitate the separation of the acid and the water as described in U.S. Pat. Nos. 5,603,812 and 5,759,357. Furthermore, the acid can be recovered by hydrogenation of the spent acid over a hydrogenation catalyst (U.S. Pat. No. 5,472,921).
It has now been found that thermal treatment results in release of the acid in its active form from spent acid, and the acid is then recovered from spent acid by direct distillation. Accordingly, this invention is a process for the recovery of fluorinated sulphonic acid including the step of subjecting the spent acid to heat treatment with stepwise or continuously increasing temperature, withdrawing at least two fractions during the heat treatment; and recovering the fluorinated sulphonic acid from each fraction.
It is desirable to keep the temperature as low as possible and to limit residence time in the distillation unit in order to avoid decomposition of the acid. In one embodiment of the invention distillation is, therefore, carried out under reduced pressure as illustrated in Example 1. In a further embodiment of the invention, distillation is performed by continuous flash distillation from a thin film in e.g. a falling film evaporator or forced film evaporator. This is illustrated in Examples 2-5 using a simplified laboratory unit of a falling film evaporator.
Vacuum distillation may be difficult to operate, in particular if volatile organic substances such as isobutane may be formed, since their condensation may require a refrigerated condenser.
In another embodiment of the invention, vacuum is replaced by a stripping agent, by which the acid is stripped off of spent acid using an inert hydrocarbon stream or other inert heated gas stream as stripping agent. Stripping is performed at atmospheric pressure or at elevated pressure to facilitate condensation of the stripping agent.